• http://www.cs.columbia.edu/sip/
    Session Initiation Protocol (SIP)
    - FAQs, background materials and lists of current work in this field.
  • http://www.firsthandtech.com/
    FirstHand Technologies - Client and server software developer for service providers and telecom equipment vendors, to address enterprise mobility markets.
  • http://www.ee.oulu.fi/research/ouspg/protos/testing/c07/sip/
    PROTOS Test Suite - A test-suite to find vulnerabilities in protocol software.
  • http://www.pulver.com/
    pulver.com - A collection of information on the protocol. Contains news, reports and an IP telephony job noticeboard.
  • http://www.sipcenter.com/
    The SIP Center - Portal with technical and market resources aimed at commercial developers. Also provides a testbed area for software and proxy development.
  • http://www.sipforum.org/
    SIP Forum - A membership based forum. Also contains a news board.
  • http://www.siptutorial.net/SIP/
    SIP (Session Initiation Protocol) Introduction - An introductory tutorial talks about protocol functions, components, message types, how it works and where it is applied.
  • http://www.toyz.org/cgi-bin/sipwiki.cgi
    SIP Wiki - Wiki based resource containing information and links.
  • http://www.sipfoundry.org/
    SIPfoundry - An international open source community providing VoIP solutions and technology based on the SIP standard.
  • http://www.softfront.co.jp/en/
    Soft Front - Provides a software development environment.
  • http://www.tech-invite.com
    Tech Invite - Portal containing links to technical information and RFC documentation.
  • http://www.vidosystem.com
    Vidosystem - Produces softphones. Site has downloads section and information on protocols supported.
  • http://www.voice-sistem.ro
    Voice System - Design, consultancy and development for SIP Express Router (SER) servers.
  • http://www.voipuser.org
    VoIP User - Consumer guide and forum. Also has software phone downloads and links to service providers.